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LDAS talk 30-6-10 MLewis 11

Telescope Thermal Effects



• The purpose of a telescope is to gather more light than the eye on its own can, and to resolve features finer than the eye can.

• For planets, and for deep-sky objects where brightness is not the limiting factor, the ability to resolve fine details is limited by three things;

• The ultimate resolving power of the telescope- a function of aperture for good optics

• The steadiness of the part of the atmosphere you are looking through

• The steadiness of the air in the telescope and your immediate surroundings


Resolving Power of the Telescope

• Stars are so far away that they should appear as point sources. However, in a telescope are not seen as points of light, but as discs. This arises from the diffraction of light from the edges of the circular aperture which is the

objective or primary mirror- Airy disc

• The larger the aperture the smaller the Airy disc, and the better the potential resolution. Resolution (arc secs) = 4.56/D (inches)

• Telescopic images of extended objects such as planets are actually made up of an overlapping pattern of Airy discs- this limits the maximum resolution for detail


Hubble Mars 2003 Aug 26

94 inch aperture 0.05” resolution

18 inch aperture 0.25” resolution 9 inch aperture 0.5” resolution 4.5 inch aperture 1.0” resolution

Best possible planetary resolution versus aperture

To improve ultimate resolution use a larger aperture scope


A still, well-defined Airy disc is a rare thing in a scope of any size above about 4-6”. Thermal effects in the atmosphere (seeing) and in the telescope blur and extend the size of the Airy disc

Video of Polaris/Arcturus in an 8 ¾” Newtonian, scope cooled for 2 hours, seeing moderate (A1 104.avi & A2 045.avi). Image approx. 2”-5” across


Astronomical ‘Seeing’• Heat haze on a hot day can destroy the clarity of a far-off view.

Astronomical ‘seeing’ is a similar process occurring at night.

• Image degradation due to atmospheric problems occurs when air of different temperatures mixes.

• The refractive index/density of air is strongly temperature dependent

• The atmosphere usually has turbulent regions where air of different temperatures/densities/refractive indices mixes non-uniformly. This causes local distortions in the column of light reaching the telescope from the object. The turbulent refraction leads to Airy disc break-up and rapid movement effects which destroy detail in astronomical images


Astronomical ‘Seeing’

• The atmosphere has large temperature variations within it but if the relative movement of the air is uniform and laminar (in layers) then this does not generally disturb astronomical images as the whole column of light is

affected in the same way.

• Turbulent mixing generally occurs in 2 areas of atmosphere

• Tropopause (~11km high) is division between Troposphere and Stratosphere. Here seeing degradation caused by turbulence at upper

surface of Jet Stream

• Bottom of Troposphere – where clouds live(~1000m high)

• Good seeing usually associated with

• high humidity- cloud, fog are signs of high humidity

• lowish wind speed

• away from Jetstream- See

http://www.weather.unisys.com/gfs/12h/gfs_4panel_12h_eur.html


300mb = ~9km


Local ‘Seeing’

• Local convection currents also have a large impact on seeing

• Stored heat from sun rising from rooftops and paved/concrete areas as cools- worst early evening

• Heat from chimneys central heating etc

• Heat rising from observer can have big effect if wind direction unfavourable


Seeing Effects on Jupiter

• Damian Peach Videos

http://www.youtube.com/watch?v=w-2F3Db6HKg&feature=player_embedded

http://www.youtube.com/watch?v=-tiIFhNVPT0&feature=player_embedded

http://www.youtube.com/watch?v=or1A4g14_jM&feature=player_embedded

http://www.youtube.com/watch?v=wrUinQRv79k&feature=player_embedded

http://www.youtube.com/watch?v=RavpiN0Axj4&feature=player_embedded


Starting to Separate out the Effects• Thermal effects can be readily seen by examining a star image defocussed

so that star is 10-30°across.

• Such a study allows one to separate distant effects from local effects.• See videos

Defocus few rings (A3 103.avi)Large defocus- seeing bands then local (A4 98.avi)

• We have little control over distant effects although of course we can chose not to observe then and save our energy for better nights.

• Local effects we are able to have some control over

• Change observing location (see A5 107.avi)

• Cover up sources of heat

• The large defocus method also allows us to see thermal effects within the telescope itself (see A6 109.avi)


Starting to Separate out the Effects• Understanding thermal effects within the telescope itself is the first step to

improving the thermal performance of the scope and so improving its resolution.

• A telescope with thermal issues will rarely perform well even on nights of very good seeing. To make the most of those rare nights of good seeing it is essential to pay attention to thermal aspects of the telescope……


• Two main problems are

• Tube Currents i.e. convection currents within tube

• Thermal Boundary Layer on objective or mirror

© Sky & Telescope Magazine



• A telescope has an internal column of air that it looks through and this column of air is generally not of uniform temperature

• Air of different temperatures has different densities with hotter air being lighter. Hotter air rises up the tube through the denser colder air.

• As we know air of different temperatures/densities also has different

refractive indices. These variations in temperature within the column of air in the telescope tube lead to non-uniform refraction. This causes distortion of the wavefront and disturbs the diffraction image leading to a loss of resolution

• A 0.25°C temperature variation of the air across a 1m long tube can render

a telescope useless (gives ½ wave delay)

• See video A7 084.avi and A8 079.avi

Tube Currents


• Air in front of cooling lens or mirror is warmed by conduction and convection and forms a thick, non-uniform and ever-changing layer in front of it called the boundary layer

• The mirror or lens looks right through this problem layer

• The boundary layer cause image deterioration in same way as tube currents affect resolution and contrast

• Once scope has cooled for a while, the boundary effects dominate over tube currents (see A9 110.avi)


Boundary Layer


• Once mirror is within 2°C of ambient then boundary layer and tube currents are effectively eliminated BUT

• Even if the scope has reached close to ambient temperature, if the air temperature is falling rapidly and the scope has a large thermal mass, this is equivalent to telescope temperature rising

• Can also get problems from warm air rising from mirror cell, secondary

holder, and for open tubes from observers body

• Also ‘Inverse Tube Currents’ where cold air is falling from inside of colder top face of tube which may be several degrees cooler than ambient

General Thermal Effects


Real cooling results after 2hrs under a clear sky

Clear with very light wind Surface Temp. Dewing

Temp. belowambient

Ambient 11.5°C

Black plastic 9.1°C 2.4°C mod. dewing

Black nylon scope shroud 9.1°C 2.4°C water drops

Aluminised mylar 10.6°C 0.9°C light dewing

Aluminium kitchen foil 11.0°C 0.5°C

Clear with strong breeze Surface Temp. Dewing

Temp. belowambient

Ambient 14.2°C

Black plastic 12.7°C 1.5°C light dewing

Black nylon scope shroud 12.6C 1.6°C

Aluminised mylar 13.9°C 0.3°C

Aluminium kitchen foil 14.0°C 0.2°C


Scope Types/Sizes• As aperture increase, then looking through greater cross-sectional area of

tube and usually a greater length of tube so tube currents get worse.

• Boundary layer problems generally related to thickness of objective/mirror rather than diameter as cooling rate dependent on thickness rather than diameter. Bigger scopes generally have thicker mirrors.

• Refractors are generally much better for thermal problems than Newtonian reflectors because;

• Smaller aperture

• Tubes are closed-ended so system is essentially sealed and cools down more slowly and uniformly

• Light only makes one pass through tube

• Light path tapers in away from tube walls where hot and cold currents tend to collect

• Long scopes with optical windows and thick mirrors are the worst of all


• SCTs are

• generally shorter-tubed than Newtonians of similar aperture which helps with tube currents

• tubes are closed-ended which is an advantage for small diameter refractors but more problematic for larger SCT scopes where the ratio of volume to area is larger.

• mirror can cool from rear as exposed inside the tube but fans rarely fitted to SCTs to remove warm air

© Roth Ritter © Roth Ritter

Scope Types/Sizes


• Allow scope to cool down before start observing- put outside for a while before observing to cool down, rather than taking straight out from centrally heated house

• Have bottom of scope tube as open as possible to allow air to freely

circulate around mirror

• Thin mirrors cool down much quicker than thick ones- big advantage of thin mirrors. Cooling rate determined by thickness rather than diameter

• Open tube with breathable fabric shroud helps tube currents

• Use of insulation or aluminium foil on top half of tube to prevent chilling and Inverse Tube Currents

Solutions


• Fan at back of mirror dramatically speeds up mirror cool-down and to keep mirror close to ambient

• Can also have fans to blow air across front of mirror and break-up boundary layer somewhat


Mirror Cooling

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